While lack of sexual attraction, lack of sexual behavior, and self-identification as asexual have been used as criteria to define asexuality, it is not known how much they overlap in describing the same group of people. This study aimed to assess how many individuals could be identified as asexual based on each of these criteria and on combinations of these criteria. Participants were recruited through the Asexuality Visibility and Education Network, social media, and posts on several health- and lifestyle-related websites. In total, 566 participants between 18 and 72 years old (M = 27.86, SD = 10.53) completed an online survey (24% male, 68.9% female, 7.1% “other”). Based on self-identification or lack of sexual attraction, 71.3% and 69.2%, respectively, of participants were categorized as asexual, while based on lack of sexual behavior only 48.5% were categorized as asexual. Gender differences were found only for those participants who indicated that they did not experience sexual attraction, with more women (72.8%) than men (58.8%) indicating a lack of sexual attraction. Given that self-identification as asexual implies familiarity with the term asexual, we argue for the use of lack of sexual attraction as the primary criterion to define asexuality.